1. Field of the Invention
The present invention relates electrical cables for transmitting electrical signals, and especially cables which transmit with high signal fidelity.
2. Description of Related Art
Traditional low signal level cable designs (also referred to as "low noise" and "low triboelectric effect" wires or cables) have used full density materials such as polyethylene or a fluoropolymer such as fluorinated ethylenepropylene (FEP) or polytetrafluoroethylene (PTFE) for their dielectric insulation. These dielectrics are then further insulated with a layer of carbon impregnated PTFE or silicon. This additional layer acts as a semi-conductive layer to attenuate charges originating in shielding for the cable.
Unfortunately, full density insulators like PTFE suffer from a high capacitance which can limit their performance in certain high demand applications, such as in high gain audio amplifiers, oscilloscope probes, piezoelectric components (e.g., microphones, accelerometers and eddy current sensors). Capacitance (C) in this instance is defined as: ##EQU1## Where .sub.eff =effective dielectric constant; D=diameter over the dielectric; and d=diameter over the center conductor. Typical capacitance values in commercially available products today range from 28-30 Pf/ft in 50 ohm designs. For many demanding applications, far better capacitance performance is desirable.
Expanded PTFE insulation, such as that which can be made in accordance with U.S. Pat. No. 3,953,566 to Gore, has many desirable properties over full density fluoropolymer insulations, including lower dielectric constant, improved matrix tensile strength, lighter weight, etc. Although expanded PTFE insulative material provides improved dielectric performance, it generally has not been used in many low signal applications because of triboelectric capacitive effect between metallic elements (e.g., cable shield and/or conductor) and the expanded PTFE insulation aggravated by the presence of air entrapped within the expanded PTFE. This condition can lead to "noisy" performance by the cable due to static charges generated by the cable under flex.
Accordingly, it is a primary purpose of the present invention to provide an improved low-noise electrical insulation which has low dielectric constant.
It is another purpose of the present invention to provide an improved electrical insulation which incorporate desirable insulative properties of expanded PTFE while maintaining structural integrity between layers of insulation.
It is still another purpose of the present invention to provide an improved electrical insulation which has a reduced tendency to generate triboelectric interference even without an added static dissipating layer.
These and other purposes of the present invention will become evident from review of the following specification.